This invention relates to polyolefin compositions containing a high concentration of an ultraviolet light absorber and a method for dispersing and stabilizing said ultraviolet light absorber at said high concentration in polyolefin compositions.
It is often desirable to incorporate an ultraviolet light absorber into a polyolefin composition to increase the stability of the composition in the presence of ultraviolet light. In such an instance the ultraviolet light absorber (U.V. absorber) is usually incorporated into the polyolefin composition at a concentration of from about 0.01 to about 1% by weight. A commonly employed method of incorporating the U.V. absorber into the polyolefin composition comprises mixing a predetermined amount of absorber with a given amount of polyolefin composition to yield a resultant composition having the concentration of U.V. absorber desired for a particular application. The mixing process is time-consuming and requires careful attention to insure that a uniform dispersion of the absorber in the polyolefin is obtained.
Another method of incorporating the U.V. absorber into the polyolefin composition utilizes a two step procedure. In the first step a concentrate is made by dispersing a high concentration of U.V. absorber in a first polyolefin composition. The concentrate is then blended with a second polyolefin composition in an appropriate proportion to achieve the desired concentration of U.V. absorber. This two step method of blending a concentrate with the second polyolefin composition more readily yields a uniform dispersion of absorber in the second polyolefin composition than the single step method of mixing the second polyolefin composition directly with the U.V. absorber.
The concentration of the U.V. absorber in the concentrate is usually from about 10 to about 60% by weight. A problem arises in making such a concentrate with certain desirable ultraviolet light absorbers due to their low viscosity at normal mixing temperatures in the polyolefin composition. The U.V. absorbers that are difficult to incorporate into a concentrate at such high levels are generally those that have a melting point below the temperature at which the absorber and polyolefin are being mixed. Particularly difficult problems arise with absorbers that melt at or below about 100.degree. C. Heretofore, concentrates made with such U.V. absorbers have had a maximum concentration of U.V. absorber of about 5-10% by weight. Prior attempts to produce a concentrate having a concentration of such an absorber above this amount have failed. This was primarily due to the migration of the absorber to the surface of the polyolefin absorber concentrate which thus prevented the production of a stable uniform dispersion of the absorber in the concentrate.
There is, therefore, a need for a polyolefin composition having uniformly dispersed therein from about 10 to about 60%, by weight, of a U.V. absorber having a melting point at or below about 120.degree.-250.degree. C and a method of making the same.